Elucidating the Mechanism of Self-Healing in Hydrogel-Lead Halide Perovskite Composites for Use in Photovoltaic Devices

Since the emergence of organometal halide perovskite (OMP) solar cells, there has been growing interest in the benefits of incorporating polymer additives into the perovskite precursor, in terms of both photovoltaic device performance and perovskite stability. In addition, there is interest in the self-healing properties of polymer-incorporated OMPs, but the mechanisms behind these enhanced characteristics are still not fully understood. Here, we study the role of poly(2-hydroxyethyl methacrylate) (pHEMA) in improving the stability of methylammonium lead iodide (MAPI, CH3NH3PbI3) and determine a mechanism for the self-healing of the perovskite–polymer composite following exposure to atmospheres of differing relative humidity, using photoelectron spectroscopy. Varying concentrations of pHEMA (0–10 wt %) are incorporated into a PbI2 precursor solution during the conventional two-step fabrication method for producing MAPI. It is shown that the introduction of pHEMA results in high-quality MAPI films with increased grain size and reduced PbI2 concentration compared with pure MAPI films. Devices based on pHEMA-MAPI composites exhibit an improved photoelectric conversion efficiency of 17.8%, compared with 16.5% for a pure MAPI device. pHEMA-incorporated devices are found to retain 95.4% of the best efficiency after ageing for 1500 h in 35% RH, compared with 68.5% achieved from the pure MAPI device. The thermal and moisture tolerance of the resulting films is investigated using X-ray diffraction, in situ X-ray photoelectron spectroscopy (XPS), and hard XPS (HAXPES). It is found that exposing the pHEMA films to cycles of 70 and 20% relative humidity leads to a reversible degradation, via a self-healing process. Angle-resolved HAXPES depth-profiling using a non-destructive Ga Kα source shows that pHEMA is predominantly present at the surface with an effective thickness of ca. 3 nm. It is shown using XPS that this effective thickness reduces with increasing temperature. It is found that N is trapped in this surface layer of pHEMA, suggesting that N-containing moieties, produced during reaction with water at high humidity, are trapped in the pHEMA film and can be reincorporated into the perovskite when the humidity is reduced. XPS results also show that the inclusion of pHEMA enhances the thermal stability of MAPI under both UHV and 9 mbar water vapor pressure.

. Carbon K-edge (left) and nitrogen K-edge (right) PEY NEXAFS spectra from 0 wt. % and 5 wt. % films with prominent features are labelled. A DFT simulated HEMA carbon K-edge spectrum is included for comparison.

Film appearance during degradation
As shown in Fig. S5, the change in appearance of the film can be observed by tracking the degradation of the film in different humidities. In about 35 % RH at RT, the pure MAPI film began to turn pale around the fifth or sixth day, marking the initial decomposition of the perovskite; while those incorporated with pHEMA maintained their initial colour throughout the observation window, due to the passivating role of pHEMA. In a more severe environment of about 70 % RH at RT, the reference sample on the fifth day showed a yellow PbI 2 phase, indicating that the MAPI film was severely degraded. The appearance of the pHEMA-incorporated MAPI film remained close to the fresh sample, with no obvious change visible to the naked eye, suggesting excellent stability.   Table S4. Elemental ratios in a 0 wt. % MAPI film as a function of temperature. Values were calculated from core level XPS spectra displayed in Fig. 1. Table S5. Elemental ratios in a 5 wt. % MAPI-pHEMA film as a function of temperature. Values were calculated from core level XPS spectra displayed in Fig. 1.

Estimation of overlayer-corrected nominal stoichiometry
The presence of a pHEMA-rich layer at the surface of the 5 wt. % films, observed in both XPS and HAXPES measurements, requires us to reconsider the stoichiometry we expect to observe in XPS and HAXPES of the pHEMA-incorporated films. For a fixed photon energy (as here), the KE of photoelectrons from the core levels used to calculate the elemental ratios is not constant, and hence the sampling depth is different for each element measured. For a homogenous sample, this can usually be easily corrected for during analysis. However, in the presence of an overlayer, photoelectrons from higher BE core levels, and therefore lower KE, experience greater attenuation, leading to an underestimation of elemental concentration in comparison with signals from lower BE core levels.
To account for the varying loss of signal due to the pHEMA overlayer we can approximate the sample to a 2-layer system, and use the flux attenuation expression shown in Equation S1 : Here, I and I 0 represent the observed and emitted photoelectron intensity from a given core level, d is the effective pHEMA overlayer thickness (calculated as ca. 3 nm at RT using inelastic background modelling of both XPS and HAXPES) and λ is the IMFP of photoelectrons from the given core level (approximated to the IMFP through pHEMA, i.e. we assume a uniform overlayer containing pHEMA only, overlying a bulk consisting of stoichiometric MAPI only). Equation 2 shows the intensity ratio of two core levels, from elements x and y.
Here, N x,0 /N y,0 is the nominal stoichiometric ratio for elements x and y, and N x /N y is the corrected ratio, taking into account the attenuation of signal from both core levels by the overlayer. σ represents the photoemission cross section for each core level and T rel is a factor correcting for the change in analyser transmission function between the KE ranges of the peaks compared. In Equation S2, the σ and T rel factors cancel to give an equation for corrected stoichiometric ratio, shown in Equation S3 .

Core level
To provide further insight into the effect of pHEMA concentration on the stability of MAPI, XPS was performed on MAPI films containing various concentrations of pHEMA (0 -10 wt. %). Data were acquired following sample fabrication (fresh), and after 14 and 21 days of storage ex situ in containers with controlled humidity. Figs. S10 and S11 display Pb 4f 7/2 , I 3d 5/2 and N 1s high-resolution core level XPS spectra acquired from films with various concentrations of pHEMA, stored in 35 % and 70 % RH respectively. No significant core level shifts are observed in samples stored at 35 % RH after 21 days; however, the 0 wt. % (pure MAPI) and 10 wt. % films each experience a +0.1 eV BE shift in Pb 4f 7/2 and I 3d 5/2 peaks position following 21 days at 70 %. However, these shifts were found to differ between different spots on the sample surface, so may only be evidence of an area of localized degradation, rather than increased degradation across the whole sample.
Elemental concentrations were determined from the intensity of peaks fitted to the core level spectra and are displayed in Figure S12, as a function of pHEMA concentration and moisture exposure. Little change in the I/Pb 2+ ratio and only a slight reduction in the N/Pb 2+ ratio is observed for all samples at both humidities. However, the concentration of Pb 0 appears to increase more rapidly in the MAPI sample compared to all concentrations of pHEMA, as observed in our in situ degradation studies.   Table S8. N/Pb 2+ ratios of 0 wt. % and 5 wt. % films as a function of electron emission angle. Values were calculated from core level HAXPES spectra. Sampling depth was approximated using the IMFP of N 1s photoelectrons passing through MAPI (14 nm), and calculated with eqn (3) in the main text. Figure S14. Inelastic background fits to the I 2p 3/2 and Pb 3p 3/2 regions of the θ = 4° HAXPES survey scan from a 5 wt. % pHEMA-MAPI sample, generated using the QUASES-Tougaard software. 5,6 The purple line corresponds to a simulated inelastic background from MAPI topped with a 3 nm overlayer of pHEMA, and the black line shows the true background.